As awareness over global environmental problems increases, as a result that exhaustion of fossil materials and petroleum resources, and increasing carbon dioxide are seen as problems, research and development of biodegradable resins such as an aliphatic polyester and resins synthesized using plants as a raw material are actively made. Of aliphatic polyesters, a polylactic acid having particularly excellent moldability is brought to attention as a resin originated from plants, using lactic acid obtained by fermentation from grain resources such as corn. However, the polylactic acid has slow crystallization rate and low heat resistance, in addition to the defect that it is hard and brittle, and therefore, had the limit on use expansion. In particular, for example, in the case of a polylactic acid amorphous molding, because a softening point is lower than 60° C., the problem was pointed out that it is liable to cause whitening, deformation and the like under daily use environment. Further, lactic acid-based resins have the defect that impact strength is poor, thus being brittle, because of its rigid molecular structure, and improvement of those properties of lactic acid-based resins is desired.
There is technology called polymer blend or polymer alloy as the technology conventionally known as an improvement method of physical properties of a resin. Various resins are forcedly mixed and kneaded, and improvement in impact resistance, flexibility, rigidity and heat resistance is tried. Some attempts to improve physical properties by mixing dissimilar resins with an aliphatic polyester are carried out. For example, JP-A-10-251498 discloses a polylactic acid-based resin composition having improved impact resistance obtained by mixing 1 to 15% by weight of a syndiotactic polypropylene with a polylactic acid. Further, JP-A-9-316310 discloses a method of improving impact resistance of a polylactic acid by mixing a modified olefin compound with the polylactic acid. Further, JP-A-2002-37987 discloses a polylactic acid composition having improved melting characteristics, mechanical characteristics and impact resistance, obtained by mixing a polylactic acid and a thermoplastic elastomer (ethylene-propylene-diene rubber).
However, a method of improving heat resistance by mixing dissimilar resins with a polylactic acid is scarcely known. In particular, an attempt to improve heat resistance by blending, for example, a polypropylene having fast crystallization rate with a polylactic acid is not known. The above JP-A-10-251498 describes that the polypropylene to be blended uses a small amount of a low crystalline syndiotactic polypropylene having 40% or lower of crystallinity. It is described that where the general high crystalline polypropylene having 60 to 70% of crystallinity is used, mixing dispersion state is poor, which is not preferable. Further, it is described that the amount of the syndiotactic polypropylene used is limited to 15% by weight or less, and when exceeding 15% by weight, a uniform composition is not obtained. Only where a low crystalline syndiotactic polypropylene in a small amount of 15% by weight or less is mixed with a polylactic acid, it is impossible to realize sufficient heat resistance. Compatibility between a polyolefin resin and a polylactic acid-based resin is extremely low, and by merely blending and kneading those, phase separation or non-compatibilization occurs, and it is difficult to obtain expected physical properties (particularly heat resistance).
On the other hand, as a method of improving impact resistance of an aliphatic polyester resin, a method of adding a modified olefin to a polylactic acid is disclosed in, for example, JP-A-10-251498. JP-A-11-124495 discloses a method of adding a copolymer of other flexible aliphatic polyester and a polylactic acid. Further, Non-Patent Document 1 (MACROMOLECULAR CHEMICAL and PHYSICS, published in 1996, vol. 197, pages 1503 to 1523) discloses a method of adding a poly(ε-caprolactone) which is an aliphatic polyester. However, improvement of impact strength by those methods is not sufficient, and it is necessary to add a large amount of modifiers for improving impact strength. This gave the problem that moldability or heat resistance deteriorates.
By the way, as a method of improving impact strength of a resin, a method of dispersing a flexible rubber in a resin is widely known. It is known that when a particle size of the rubber dispersed in the resin is about several μm or smaller, such is effective for the improvement of impact strength. However, it is generally difficult for two kinds of polymers to compatibilize with each other, and as a result, a particle size of the rubber added to the resin becomes very large, and impact strength is not improved. Dispersion state of the rubber can be greatly improved by adding a compatibilizer which improves compatibility between two kinds of polymers, thereby decreasing interfacial tension between dissimilar polymers.
A compound having two kinds of polymers to be compatibilized as the respective block is considered to have excellent effect as a compatibilizer for improving dispersion state of a rubber, and Non-Patent Document 2 (JOURNAL OF APPLIED POLYMER SCIENCE, published in 2003, vol. 89, pages 3757 to 3768) describes that impact strength of a polylactic acid is greatly improved by adding a block polymer of a polylactic acid and a polyethylene to a blend of a polylactic acid and a linear low density polyethylene. However, there was the disadvantage that the block polymer of a polylactic acid and a polyethylene involves complicated polymerization method, and this is liable to increase the cost.
Further, a rubber can be finely dispersed in a resin by improving compatibility between the rubber to be finely dispersed and the resin. One method of improving compatibility between a rubber and a resin is a method of adding a site having good compatibility with a resin to a rubber. For example, it is described in Non-Patent Document 3 (Polymer ABC Handbooks, Polymer Society, edited by Polymer ABC Research, published Jan. 1, 2001, pages 372 to 379) that in a rubber-reinforced polystyrene (HIPS), impact strength is improved by grafting styrene on a rubber, and dispersing the same in a styrene resin. However, differing from a vinyl polymer such as a styrene resin, in an ester condensation polymer such as a polylactic acid resin, it was difficult to produce a block copolymer, a graft copolymer or a random copolymer, with a rubber effective for improving impact strength.